The general guidelines in respect of laser safety (for example in accordance with EN 60825, ANSI Z136 or corresponding to the “International Commission on Non-Ionizing Radiation Protection”) demand that a laser light source can only be operated such that it does not pose any hazards. In this case, depending on the wavelength emitted, limit values for the thermal power density or the energy density need to be adhered to. Eye safety is of primary importance in laser devices since the eye, owing to its function, reacts most sensitively to electromagnetic radiation and damage to the retina or cornea can occur as a result of direct, indirect or scattered laser beams. This applies in particular since studies have shown that, even in the case of visible light, it generally cannot be assumed that the lid closure reflex protects the eye. Laser devices need to be classified corresponding to these specifications and to be identified correspondingly, which could also include safety testing and certification.
In the case of a large number of laser devices, for example electro-optical measurement devices such as laser distance measuring devices or laser levelers, laser projectors, laser scanners, etc., the emergence of the laser light from the housing of the device is absolutely necessary for functional reasons. A minimum optical energy is also often required for functional reasons, and this minimum optical energy would be above the nonhazardous limit if this were to be emitted in the continuous-wave operating mode. Therefore, the lasers in such devices are usually operated in pulsed fashion. In order to guarantee eye safety, the laser sources need to be equipped with corresponding protective measures in order to adhere to relevant standards and specifications. This applies not only in the conventional operating case, but also under so-called “single-fault conditions”, which all cover any fault scenarios in which a single fault occurs on its own (for example failure of individual component parts, short circuit, conductor track breakage, etc.). In this case, even in the event of failure of any component, for example, it is necessary to ensure that, despite the occurrence of such a single fault, the laser power occurring falls to below the upper power limit of the corresponding laser class within fractions of a second in order to rule out damage to the eye. During pulsed operation, care should be taken, using special measures, to ensure that continuous emission with the peak pulse values which are above the limit values can also be ruled out in the event of a fault. Such single-fault tests need to be correspondingly carried out and verified by the manufacturer of the laser device.
Depending on the laser class and the application case, two-fault or multiple-fault failsafety can also continue to be required, in which, even in the event of the simultaneous occurrence of more than one fault, evidence needs to be given of the safety to the effect that the emitted laser power does not in any way exceed the limit values and therefore a risk to the user can be ruled out. The single-fault failsafety described in this document also represents a basic precondition which in any case needs to be met in these cases, which basic precondition can be developed by corresponding further measures.
The conventional solution for producing laser safety is the direct monitoring of the emitted laser power with the aid of a monitor diode. In this case, some of the emitted laser light is directed onto a photosensitive element, for example a photodiode or a phototransistor, which provides an electrical signal which is dependent on the light intensity. A monitoring circuit, for example in the form of a microcontroller or a discrete circuit (possibly even a mandatory one in the case of safety-relevant circuit parts), can thus monitor the presently emitted laser power and possibly disconnect the laser in the event of a limit value for the emitted laser energy or laser power being exceeded.
For example, documents EP 0 314 390, U.S. Pat. No. 5,287,375, EP 0 780 937, EP 0 664 591 and EP 0 597 644 disclose a wide variety of laser driver circuits which all have a monitoring circuit with a monitor diode, to which some of the light emitted by the laser diode is applied and with the aid of which the present output power of the laser diode can be determined. In the event of a fault, for example in the event of a short circuit of a power transistor in the output stage of the laser diode actuation, the supply of electrical energy to the laser diode can be reduced or suppressed on the basis of this information by means of a correspondingly designed safety circuit.
Owing to the general nature of the laser safety provisions or else owing to regulation of the output power of the laser diode which is often required depending on the application, a large number of commercially available laser diode components are already equipped with a corresponding monitor diode. Laser diodes also often have considerable manufacturing tolerances, temperature dependencies or ageing effects, which can be compensated for by determination of the actual optical output power. The circuitry complexity for such monitoring is really high and also correspondingly cost-intensive. The complexity of evaluating the analog and often interference-susceptible signal of the photosensitive monitoring element, often in the form of a photodiode, thus remains. Often such a safety circuit also needs to be implemented in a manner which is safe and certifiable, supported by corresponding evidence, which can additionally increase the complexity in terms of circuitry, in particular since the realization of safety-relevant circuit parts is often linked to stringent conditions and documentation specifications.
In addition, during pulsed operation of laser diodes, during which only very short light pulses with a high peak power are emitted, owing to the steep switching edges occurring in the process in the electronic circuit and high peak currents, corresponding electromagnetic interference signals (crosstalk, etc.) are to be expected, which disrupt the evaluation (which usually takes place at high resistances) of the measurement signals of a monitor diode and can complicate this. The short pulse duration of the emitted light and the corresponding monitor signals can also make monitoring of the actually emitted average light energy more difficult.
In this case, the laser pulses are emitted multiply in packets of pulses, so-called “bursts”, as is described, for example, in EP 01 957 668. After a packet with a number of short laser pulses in quick succession, there is a dead time in which no laser emission takes place and which is markedly longer than the intervals between the pulses within the packet.
JP 7 079042 discloses the use of a pulse-shaping network in order to supply a current pulse which does not have any interference to a laser diode.
JP 2008 227408 describes an energy-efficient increase in a DC voltage by means of a step-up converter for supplying power to a series circuit comprising a plurality of light-emitting diodes with a forward voltage which is greater than the DC voltage.
One object of the present invention therefore consists in improving the actuation of a laser diode, in particular while adhering to safety specifications.
One object also consists in providing a safe laser driver for pulsed or burst operation, which can be realized more easily and at less expense and has, for example, decreased complexity in terms of components and evaluation.
A subordinate object is in this case also providing a laser driver which can be produced in highly integrated form, i.e. in addition to a conventional printed circuit board design also using thick-layer technology, thin-layer technology or an ASIC, for example.
A further object consists in achieving laser safety using a circuit in which no analog monitor signals of a photosensitive element are evaluated in the safety circuit, in particular in which no analog-to-digital conversion (ADC) is required.
A subordinate object also consists in actuating a laser diode, in accordance with safety standards, which has fewer electrical connections than a laser diode equipped with a monitor diode, in particular a two-pole laser diode with an SMD design.
The development of a laser driver circuit with single-fault failsafety which manages without a monitor diode is also an object of the present invention.
Likewise, the provision of a laser driver as an ASIC, in particular with a purely digital interface to the outside, is an object of the present invention.
Furthermore, a safe laser driver according to the invention for a laser diode of an electro-optical distance measuring device for emitting laser light pulses with a laser diode voltage supply, for providing a voltage which is below a laser threshold voltage of the laser diode is described. As a result, during steady-state application of the voltage at the laser diode, no laser light can be emitted.
The laser driver also has an inductive component in a supply path of the laser diode, in particular electrically connected to the laser diode, and an electronic switching element.
In this case, the switching element is arranged interactively in such a way that, in a first steady-state switching position of the switching element, a current flow through the inductive component can be produced and, in a second steady-state switching position of the switching element, the current flow can be conducted through the laser diode, especially with the inductive component in the circuit between the voltage supply and the laser diode and in particular with a switching element connected electrically in parallel with the laser diode.
Only in the event of a change from the first to the second switching position can one of the laser light pulses be emitted. A single one of the laser light pulses which can be emitted in the event of a change from the first switching position to the second switching position has an energy which is limited by the energy stored in the inductive component owing to the current flow, and also the average emitted laser power of a group of laser light pulses is thus limited.
In particular, the laser driver can thus have a safety circuit without any monitor diodes for maintaining a maximum permissible average laser power.
Therefore, a safety circuit without any monitor diodes can thus be used to ensure that a maximum permissible emission power corresponding to laser safety guidelines is adhered to, specifically also in the single-fault case, for example in the case of failure or short circuit of a component or element of the laser driver.
A basic concept of the present invention consists in that the DC forward voltage used to supply the laser diode is below the laser threshold of the laser diode. This means that the laser diode cannot emit laser light in the DC case. If in the case of direct application of the supply to the laser diode (for example in the event of a short circuit), there is no stimulated or intensified emission, but in any case only a weak, spontaneous, non-intensified emission of photons, similar to a light-emitting diode. Thus, in this operating or fault state, the laser safety is ensured.
In order nevertheless to be able to emit laser light during pulsed operation, in an exemplary embodiment used for explanatory purposes, an inductively acting component is in series with the laser diode and a switching element by means of which the laser diode can be short-circuited is in parallel with the laser diode. The sequence for the emission of a laser pulse is as follows:
1. A pulse generator switches the switch in parallel with the laser diode. As a result, the supply voltage (which is below the laser threshold, as described) is present at the inductive component.
2. A current is impressed in the inductive component. This current flows from the voltage source via the inductive component and the switch to ground.
3. The pulse generator opens the switch.
4. Since the current in the inductive component, despite the open switch, nevertheless continues to remain, this current now needs to flow through the laser diode. The potential at the anode of the laser diode is elevated to above the laser threshold and the stimulated emission of a laser pulse begins. The maximum energy of the pulse is in this case restricted by the energy stored in the inductive component, however.
If the current has decreased and the voltage at the anode of the laser diode has decreased to below the laser threshold voltage, the laser emission is also at an end.
5. The process can be repeated with step 1.
The laser pulses can therefore be emitted as individual pulses or in pulse packets (bursts) or as a continuous pulse sequence. The switch can remain closed or open between the pulses or bursts. In both cases, the laser will not emit any light.
If the switch is open, a possible current in the inductive component decreases via the laser diode, and the voltage across the laser diode falls to below the threshold (approximately to the level of the supply voltage). Since the supply voltage is below the threshold voltage, in the case of a switch which is open in the steady state, no laser light is produced. Therefore, in this steady operating state, no laser emission is possible.
If the switch is closed, the current flows through the switch and not through the laser diode, as a result of which, in this steady operating state, likewise no laser emission is possible. The laser diode is short-circuited by the switch, and therefore the voltage and the current at the laser diode are approximately zero, in any case below the threshold values.
Only in the event of a change from the closed switch to the open switch is emission of the laser diode possible, but in this case also with a defined upper limit for the possible pulse energy. In no case is continuous-wave emission of laser light possible.
The laser power can be adjusted via the number of pulses per burst or the pulse repetition rate within the burst. For example, this can be determined or adjusted during production of the laser or assembled laser device in the context of function testing or calibration. The energy of an individual pulse can also be influenced by means of adjusting the duty factor of the pulse generator during pulse generation. For example, the pulse energy can also be influenced via the durations of the two switching states during “charging” and “discharging” of the inductance. However, it is not possible for a maximum individual pulse energy to be exceeded which is determined from the maximum current when the switch is closed in the steady state and the amount of energy stored in the inductance, associated therewith. If, for example, the parameters of the pulse generator are limited on the basis of a worst case scenario, the laser safety limits being exceeded can in any case be ruled out. Thus, the laser driver according to the invention can be designed such that laser safety can be guaranteed in any case.
Ageing effects merely make the laser less efficient, and therefore the laser power will decrease slightly over time, but in no case will it exceed the limit set during production. An ageing-related power drop could, if necessary, also be compensated for by renewed determination of the actually emitted power and adjustment of the pulse or burst parameters.
In a further embodiment, the supply voltage of the laser driver or the entire system can very easily be above the laser threshold voltage. As a result, a power supply to the peripheral logic circuits, processors, communication modules, etc. with voltages which are conventional for this, and are often above the laser threshold, is enabled, for example. A step-down converter or series regulator (LDO) can in this case supply a correspondingly lower voltage, which is regulated and is independent of the supply, and which is below the laser threshold voltage, to the laser diode or the inductive component connected upstream thereof. The laser driver can in this case also be in the form of an ASIC, wherein the LDO can either be arranged externally or can be integrated directly in the ASIC.
In such an embodiment in which voltages above the laser threshold are provided in or around the laser driver, however, an additional fault case is possible, in which the safety would no longer be ensured purely on the basis of the above-explained circuit or method according to the invention, namely a short circuit between the relatively high supply and an electrical grid in the supply of the laser diode.
Such a short circuit can be identified by additional monitoring of the laser diode supply voltage. For example, this can be identified by a short-circuit detection circuit (SCD) and signaled to a superordinate microcontroller by means of a digital signal or supplied directly to a safety shutdown. The SCD can in this case also be integrated directly in the LDO and/or the laser driver, for example, especially the system deviation occurring at the LDO could be used for establishing an overvoltage, for example. This can, in the event of a fault, isolate the now excessively high forward voltage from the inductive component and/or the laser diode by means of an additional (independent) switching element. Alternatively, the additional switching element can also be used to isolate the entire laser driver from the supply voltage which is above the laser threshold. In this case, an additional safety device to protect against automatic reconnection may be provided.
Alternatively, the laser diode supply voltage can also be monitored by a voltage measurement, comparator circuit or the like. Even with respect to the additional shutdown in the event of a fault, there are functionally equivalent alternative solutions with which the required safety conditions can likewise be met.